This invention relates to a Coriolis flowmeter having a balance bar that can be subjected to a wide range of thermal conditions without applying stresses to the flow tube to which the balance bar is coupled.
Single straight tube Coriolis flowmeters traditionally have a concentric balance bar that is coaxial with the flow tube. The balance bar vibrates 180 degrees out of phase with respect to the flow tube to counterbalance the drive mode vibration of the flow tube. The balance bar and the material filled flow tube comprise a dynamically balanced structure that vibrates at its resonant frequency. The ends of the balance bar are rigidly affixed to the flow tube via annular brace bars. Regions of no vibration, called nodes, are located in the brace bars and define the ends of the active portion of the flow tube.
The radial distance between the outer surface of the flow tube and the inner surface of the balance bar is traditionally kept small both for reasons of compactness and for tuning the resonant frequency of the balance bar. The small difference in diameter between the flow tube and the balance bar results in a connection that is very rigid.
A problem with prior art designs of balance bars is that they impose a significant thermal stress on the flow tube. There are three distinct types of thermal stress of a Coriolis flowmeter. The first is thermal shock. If a Coriolis flowmeter in a cold climate suddenly receives a hot material, the hot flow tube attempts to expand, but is restrained by the surrounding cold balance bar and flowmeter case. Prior art designs use a titanium flow tube having a low modulus of elasticity. The low thermal expansion rate and the high yield strength of titanium enable the flow tube to bear the high stress of thermal shock without damage.
The second type of thermal stress is that due to an elevated or lowered uniform temperature of the Coriolis flowmeter. This thermal stress is common in chemical or food plants where Coriolis flowmeter cases are insulated or heated so as to maintain the entire meter at the material temperature. If the entire Coriolis flowmeter were titanium, a uniform meter temperature would not result in any thermal stresses, but titanium is too expensive to use for the entire meter. Most prior art Coriolis flowmeters have a titanium flow tube because of its low expansion and low modulus of elasticity. For cost reasons they have a stainless steel balance bar and case even though titanium would be the preferred material. Thermal stress is produced in these Coriolis flowmeters at elevated uniform temperatures because these different materials have different moduli of expansion. A Coriolis flowmeter that is stress free at 70 degrees has significant stresses at a uniform 200 degrees because the stainless steel balance bar and case expand at more than twice the rate of the titanium flow tube.
In the third type of thermal loading, stress is imposed on the flow tube by a steady state thermal condition in which the material and the environment have different temperatures. A Coriolis flowmeter measuring hot material in a cold climate eventually reaches a state of thermal equilibrium in which the titanium flow tube reaches the material temperature while the balance bar is only slightly cooler. The case, however, can be much cooler depending on the ambient conditions. If the case is exposed to a cold wind, for example, the case temperature may be only a few degrees above the ambient temperature. Stresses are generated when the cool case restrains attempted expansion by the balance bar and the flow tube. Stresses are also generated when the stainless steel balance bar attempts to expand at twice the rate of the titanium flow tube.
Commercially available single straight tube flow Coriolis flowmeters must be able to withstand all three types of thermal loading without suffering permanent damage and ideally without excessive error in the material measurement. The balance bar ends are rigidly affixed to the flow tube via brace bars. This effectively divides the flow tube into three portions. The central portion, between the brace bars and within the balance bar is the active portion of the flow tube. This portion vibrates out of phase with respect to the balance bar. The two portions of the flow tube that extend from the ends of the balance bar to the case ends do not vibrate and are the inactive portions of the flow tube.
When the above described prior art Coriolis flowmeter is exposed to the first type of thermal loading, thermal shock, both the active and inactive portions of the flow tube experience the same thermal stress. This is due to the fact that neither the balance bar, which constrains the active portion of the flow tube, nor the case, which constrains the inactive portions of the flow tube change temperature or length and the three portions of the flow tube quickly attain the same elevated temperature as the material and have the same thermal stress. When the prior art Coriolis flowmeter is exposed to the second type of thermal loading in having a uniform elevated temperature, the three portions of the flow tube once again experience the same thermal stress. The balance bar and case are both stainless steel and expand at the same rate. The titanium flow tube, attempts to expand at a different rate but is restrained by the balance bar and case.
Under the third thermal condition of thermal loading, the flow tube and the balance bar nearly attain the material temperature while the case remains cold. The hot balance bar expands its length while the cold case does not. The inactive flow tube portions are between the case ends and the lengthening balance bar. The balance bar and case both have much larger cross section areas than the flow tube and force the inactive portions of the flow tube to decrease in length. Since the inactive flow tube portions are hot and if unconstrained would be increasing in length, the forced decrease in length results in stress that can even exceed the yield strength of the titanium flow tube. Meanwhile, the active portion of the flow tube is constrained at its ends by the connections to the hot stainless steel balance bar. Stainless steel has a much greater expansion coefficient than the titanium of the flow tube. Depending on the temperature differential between the balance bar and the flow tube, the active portion of the flow tube could be put in tension since the balance bar temperature is nearly equal to the flow tube temperature. It could also be put in compression as when the balance bar temperature is lower than the flow tube temperature.
The situation in which the inactive portion of the flow tube is highly stressed by temperature gradients is a problem with prior art flow Coriolis flowmeters. The problem is generally solved in prior art Coriolis flowmeters by limiting the temperature range over which the Coriolis flowmeters may be operated. This is undesirable since many customers would like to measure material flow rate at temperatures that exceed the limits dictated by thermal stress.
The present invention overcomes the above and other problems by use of a balance bar that allows the stresses in the active and inactive portions of the flow tube to be as low as possible for any thermal condition. The balance bar has a middle segment that is compliant in the axial direction so that changes in length of the balance bar ends do not impose a significant axial force on the flow tube. This ensures that the thermal stresses on the active and inactive portions of the flow tube are always equal. This state of stress equality is the lowest possible stress state for the flow tube. As a result of the axially compliant balance bar, the remaining stress in the flow tube is only a function of the differential expansion between the flow tube and the case. Balance bar expansion and contraction is eliminated and has no impact on the flow tube stress.
A further advantage of the balance bar of the present invention is cost. Most prior art Coriolis flowmeters require a stainless steel balance bar to keep the cost reasonable. In order to extend the temperature range of a Coriolis flowmeter, the balance bar of the prior art is required to have an expansion coefficient as near as possible to that of the flow tube material (titanium). The best balance bar of the prior art would be one made entirely of titanium. However, the cost of a titanium balance bar in larger sized Coriolis flowmeters can be as much as six times that of a stainless steel balance bar. The balance bar of the present invention has an increased axial compliance that does not impose axial forces on the flow tube. The balance bars thermal expansion is of no concern and thus can be made of less expensive material and have a wide temperature range.
There are several possible exemplary embodiments of the present invention. A first embodiment is a balance bar having two independent end portions and a void for a center portion. Each end portion is fastened to a respective brace bar and, via the brace bars, to the ends of the active portion of the flow tube. The independent balance bar end portions behave as cantilever beams that are designed to have the resonant frequency of the material filled flow tube. The void enables the lowering of the balance bar drive mode frequency to that of the flow tube without the added balance bar mass of prior art meters. It does this by removing stiffness from the balance bar. This dynamically balances the Coriolis flowmeter. The driver comprises a drive coil that is fastened to the case because of the void in the central portion of the balance bar, and a magnet fastened to the flow tube. The independent balance bar end portions are passively driven by the motion of the brace bars in response to the drive mode vibration of the flow tube. The independent balance bar end portions respond to the drive mode vibration of the flow tube and apply a torque to the brace bar regions that counters the torque applied to the brace bars by the ends of the active portion of the flow tube. The deflection of the balance bar end portions also counter the momentum of the vibrating flow tube.
This balance bar design has an added benefit beyond reduced cost and extended temperature range with no resulting stress on the flow tube. A balance bar in prior art single tube flowmeters has been able to counterbalance the vibration of the flow tube in the drive mode, but it does not balance the vibration of the flowmeter caused by the Coriolis forces applied to flow tube during conditions of material flow. Coriolis forces and deflections are applied to a vibrating flow tube with material flow. The two axial halves of the flow tube have applied Coriolis forces of opposite directions. The resulting Coriolis deflections of the two axial halves of the flow tube are also in opposite directions. These forces and deflections are proportional to the material flow rate and they generate vibrations that cannot be counterbalanced by fixed weights on a traditional balance bar.
The balance bar of the present invention is able to counterbalances these Coriolis forces because of the independence of its two end portions. The void in the center of the balance bar lowers the resonant frequency of the balance bar end portions in a mode in which they vibrate out of phase with each other. This mode is referred to as the Coriolis-like mode because of its shape. The void lowers the resonant frequency of this mode to below the drive frequency. Each balance bar end portion resonates out of phase with the flow tube in the drive mode frequency of vibration. Because Coriolis deflections of the flow tube occur at the drive mode frequency, the two independent balance bar end portions respond to these Coriolis deflections as readily as to the drive mode deflections. The driving force for these two responses is the same. It is the motion of the brace bars. The left balance bar end portion has the same response to the Coriolis excitation as it does for the drive mode excitation. The difference between the two excitation modes is that the drive mode excitation is of a constant amplitude and the two ends of the active of the flow tube are in phase with each other. The Coriolis excitation has an amplitude that is proportional to the flow rate and the two ends of the active portion of the flow tube are 180 degrees out of phase with each other. The independent balance bar end portions have Coriolis-like deflections that effectively counterbalance the Coriolis forces of the flow tube. The out-of-phase Coriolis-like deflections increase the amplitude of vibrations of the balance bar and are out of phase with the Coriolis vibrations of the flow tube as the flow rate (and thus the Coriolis force) increases.
The counterbalancing of the Coriolis force vibrations by the balance bar end portions produces a more accurate Coriolis flowmeter. The unbalanced Coriolis forces of prior art Coriolis flowmeters result in a shaking of the Coriolis flowmeter at the drive mode frequency. This shaking, which is proportional to flow rate alters the Coriolis acceleration of the material flow and the resultant output signals of the pick offs. Compensation could be made for this error except that it is dependent upon Coriolis flowmeter mounting stiffness. A Coriolis flowmeter with rigid mount would have a slight error while a Coriolis flowmeter with a soft mount would have greater error. Since the mounting conditions of a Coriolis flowmeter in commercial use are unknown, it is generally not possible to compensate for them.
An alternative embodiment of the invention has balance bar end portions that are weakly coupled by drive coil brackets. These brackets allow the driver to be mounted in the axial center so that the coil and magnet of the driver can drive the balance bar end portions and flow tube in phase opposition. These brackets are made of sufficiently thin metal and their geometry is such that they allow the balance bar end portions to expand and contract axially with little resistance.
These flexible brackets also allow for the out of phase motion of the two balance bar end portions that counterbalance the applied Coriolis forces.
Another alternative embodiment allows for expansion and contraction of the two balance bar end portions, but does not allow for an out of phase motion of the two end portions. This permits the use of an inexpensive balance bar material along and provides a high temperature range. This embodiment does not allow for the out of phase motion of the balance bar end portions that counterbalances the Coriolis forces.
Yet another embodiment provides a balance bar with independent end portions coupled to a center section by flexible side strips. Cutouts in the center section and balance bar halves increase axial compliance.
In summary, the present invention solves three balance bar problems by decoupling the two end portions of the balance bar. It allows the balance bar to be made of less expensive materials. It allows for a wider temperature range with less axial stress on the flow tube and it provides a more accurate Coriolis flowmeter by counterbalancing the Coriolis forces applied to the flow tube.
An aspect of the invention is a Coriolis flowmeter adapted to receive a material flow at an inlet and to extend said material flow through flow tube means to an outlet of said Coriolis flowmeter; said Coriolis flowmeter also includes:
a balance bar positioned parallel to said flow tube means;
brace bars coupling ends of said balance bar to said flow tube means;
a driver that vibrates said flow tube and balance bar in phase opposition;
pick off means coupled to said balance bar and to said flow tube means to generate signals representing the Coriolis response of said vibrating flow tube means with material flow;
a first end portion of said balance bar extending axially inward from a first one of said brace bars towards a mid-portion of said balance bar;
a second end portion of said balance bar extending axially inward from a second one of said brace bars towards said mid-portion of said balance bar; and
an axial mid-portion of said balance bar having a compliance that enables said balance bar to expand and contract axially without imparting any axial stress to said flow tube.
Another aspect is that said mid-potion of said balance bar is a void.
Another aspect is that said flow tube means comprises a straight flow tube.
Another aspect is that said driver is positioned proximate said mid-portion and is coupled to an exterior surface of said flow tube and an inner wall of said case.
Another aspect is that a magnet of said driver is affixed to said exterior surface of said flow tube and a coil of said driver is coupled to said inner wall of said case.
Another aspect is a Coriolis flowmeter adapted to receive a material flow at an inlet and to extend said material flow through flow tube means to an outlet of said Coriolis flowmeter; said Coriolis flowmeter also includes:
a balance bar positioned parallel to said flow tube means;
brace bars coupling ends of said balance bar to said flow tube means;
a driver that vibrates said flow tube and balance bar in phase opposition;
pick off means coupled to said balance bar and to said flow tube means to generate signals representing the Coriolis response of said vibrating flow tube means with material flow;
a first end portion of said balance bar extending axially inward from a first one of said brace bars towards a mid-portion of said balance bar;
a second end portion of said balance bar extending axially inward from a second one of said brace bars towards said mid-portion of said balance bar;
an axial mid-portion of said balance bar;
said mid-portion comprises:
drive coil bracket means;
spring means oriented substantially perpendicular to the longitudinal axis of said flow tube and coupling said drive coil bracket means to the axial inner extremities of said end portions of said balance bar, said spring means having an axial compliance that enables said end portions of said balance bar to change in length without imparting any axial stress to said flow tube exclusive of the stress associated with the force required to flex said spring means as said length of said end portions change.
Another aspect is that said spring means flex as the axial length of said end portions of said balance bar changes with the only resultant axial stress imparted to said flow tube being the stress required to flex said springs means.
Another aspect is that said flow tube means comprises a single straight flow tube.
Another aspect is that said balance bar is co-axial with said flow tube.
Another aspect is that said pick off means comprises a pair of velocity sensors with a first one of said pick offs being coupled to said first end portion of said balance bar and to said flow tube and with a second one of said pick offs being coupled to said second end portion of said balance bar and said flow tube.
Another aspect comprises a case enclosing said flow tube and said brace bars and said balance bar.
Another aspect is that:
said material flow through said vibrating flow tube imparts Coriolis deflections to said flow tube;
said material flow through said vibrating flow tube imparts Coriolis-like deflections to said first and second end portions of said balance bar that are in phase opposition to said Coriolis deflections of said flow tube.
Another aspect is that said first and second end portions of said balance bar vibrate independently in phase with each other for drive mode vibrations imparted to said flow tube by said driver.
Another aspect is that said first and second end portions of said balance bar vibrate out of phase with each other for said Coriolis-like deflections imparted to said balance bar by said Coriolis deflections of said flow tube.
Another aspect is that a first end of said spring means is coupled to said drive coil bracket means;
a second end of said spring means is coupled to the axial inner extremity of said end portions of said balance bar;
said spring means flexes in response to said axial changes in length of said end portions of said balance bar.
Another aspect is that said drive coil bracket means comprises:
a drive coil bracket having a flat surface parallel to a longitudinal axis of said flow tube;
a second bracket having a surface parallel to said longitudinal axis of said flow tube;
said spring means comprises a first set of springs coupling said first drive coil bracket to said axial inner extremities of said end portions of said balance bar;
said flat surface of said first drive coil bracket is adapted to receive a coil of said driver;
a drive magnet is coupled to said flow tube and in magnetic communication with said drive coil;
said spring means further comprising a second set of springs coupling said second said drive coil bracket to said axial a inner extremities of said end portions of said balance bar; and
a mass affixed to said flat surface of said second bracket.
Another aspect is that said springs of said first and second set have ends coupled to said inner axial extremities of said balance bar end portions.
Another aspect is that:
said drive coil bracket means is coaxial with said flow tube and has an axial length less than the distance between said axial inner extremities of said balance bar end portions;
elongated support bars couple said axial inner extremities of said balance bar end portions to the axial outer extremities of said drive coil bracket means;
said elongated support bars are positioned in a vibrationally neutral plane of said balance bar and are oriented parallel to said longitudinal axis of said flow tube;
slots are in the walls of said drive coil bracket means, said slots are parallel to and proximate said outer axial extremities of said drive coil bracket means;
the wall material of said drive coil bracket means between said slots; and said outer axial extremities of said drive coil bracket means define a first set of springs that flex in response to changes in the axial length of said balance bar end potions;
Another aspect is that:
circumferentially oriented slots are in the walls of said balance bar end portion proximate said axial inner extremities of said balance bar end portions;
the wall material between said slots and said balance bar end portions; and define a second set of springs that flex axially in response to changes in the axial length of said balance bar end potions and in response to changes in the length of said flow tube.
Another aspect is that:
said support bar and set first and second set of springs define springs that flex in response to changes in the axial length of said balance bar end potions without imparting axial stress to said flow tube in excess of the stress associated with the force required to flex said first and second set of springs and said support bar.
Another aspect is that:
a top portion of said drive coil bracket has a flat surface with an opening for receiving a coil of said driver;
a magnet of said driver is in electromagnet communication with said drive coil and is coupled to said flow tube.
Another aspect is that:
said drive coil bracket means is cylindrical and has a diameter substantially equal to the diameter of said balance bar.
Another aspect is that said drive coil bracket means comprises:
a first drive coil bracket affixed to a top portion of said first balance bar end portion proximate said inner axial extremity of said first end portion;
a second drive coil bracket affixed to a bottom portion of said second balance bar end portion proximate said inner axial extremity of said second end portion;
spring means oriented substantially perpendicular to said longitudinal axis of said flow tube that couples said first drive coil bracket to said second drive coil bracket;
said spring means is adapted to flex about its end in response to changes in the axial length of said end portions of said balance bar;
said spring means having a flexibility that enables said end portions of said balance bar to change in length to change in length without imparting a stress to said flow tube in excess of a stress associated with the force required to flex said springs.
Another aspect is that said spring means comprises:
a first end of said spring means coupled to said first drive coil bracket;
a second end of said spring means coupled to said second drive coil bracket.
Another aspect includes a first mass affixed to a lower portion of said inner axial extremity of first end potion of said balance bar;
a second mass affixed to an upper portion of said inner axial extremity of said second end potion of said balance bar.
Another aspect is that said driver comprises:
a first drive coil affixed to a surface of said first drive coil bracket;
a first magnet in magnetic communication with said first drive coil and affixed to said flow tube;
a second drive coil affixed to a surface of said second drive coil bracket;
a second magnet in magnetic communication with said second drive coil and affixed to said flow tube;
said drive coils being and said magnets being effective in response to the receipt of a drive signal coils for vibrating said flow tube and said balance bar in phase opposition.